In the hot-dip coating of metal strip, especially steel strip, the strip being coated is guided through the metal melt by means of a deflecting roller arranged in the melt bath. In addition, the section of strip emerging from the melt bath is usually guided by guiding rollers likewise arranged in the melt bath (so-called stabilization rollers or passline rollers), in order to ensure a substantially vertical and vibration-free running of the strip through a gap bounded by stripping nozzles. The stripping nozzles separate excess coating material from the strip or adjust the thickness of the layer of coating material adhering to the strip.
The deflecting rollers and guiding rollers used in hot-dip coating plants of the prior art are designed as solid-material rollers, open shell rollers or closed hollow rollers.
Closed hollow rollers still find application today, but they can only be operated with expensive ventilation equipment, since due to the high temperatures in the melt bath there may occur very high internal pressures in the closed roller cavity, even to the point of an explosive failure, especially if there are undesirable liquid inclusions in the roller cavity.
Due to the circumstance that the deflecting and guiding rollers in the melt bath are not normally provided with their own drive system, but instead are rotated by the movement of the strip wrapped around and lying against them, the use of solid-material rollers is a problem especially in the case of the hot-dip coating of thin strips. Due to the large weight of solid-material rollers, a rotation of the rollers by the strip is difficult, especially for thin strips, which may result in relative movement (slippage) between strip and roller. This may cause scratches and other coating flaws, which reduce the quality of the coating.
Shell rollers usually consist of a hollow cylindrical shell pipe, which is connected to the bearing journals at its two ends by spoke-like webs or recesses, such as end plates having boreholes. Melt bath rollers designed as shell rollers have the advantage of being easier to rotate as compared to the heavy solid-material rollers and have no enclosed cavities as compared to the enclosed hollow rollers in which a critical internal pressure can occur. But on account of the openings at the end faces, a pumping effect may occur during the rotation of the shell rollers, resulting in a turbulent flow of the metal melt in the region of the spoke-like webs or the recesses of the end plates, resulting in a very substantial material abrasion (wear) in the region of the webs or end plates. For this reason, the service life of shell rollers used in hot-dip coating plants is relatively short.
What is common to the mentioned designs of deflecting and guiding rollers is that their rotary bearings are usually designed as plain bearings, and the bearing journals of the rollers (as well as the abutments receiving the bearing journals) are subjected to very large abrasion influences. The great wear on the bearing journals is principally due to the high bearing forces and the corrosiveness of the metal melt.
Rollers of this kind are known for example from the patent applications US 2007/0074657 A1, DE 37 18286 A1 or WO 2012/136713 A1.
Thus a need exists for a roller with which a good coating quality can be achieved in hot-dip coating of metal strip, especially steel strip, and which at the same time affords relatively long service life.